1. Technical Field
The present invention relates to spark plugs for internal combustion engines.
2. Description of the Related Art
Generally, a spark plug is employed as igniting means in an internal combustion engine of, for example, a motor vehicle. The spark plug includes a center electrode, a tubular insulator having the center electrode inserted and held therein, a tubular metal shell (or housing) having a distal part of the insulator inserted and held therein, and a ground electrode that is joined to a distal end of the metal shell and faces the center electrode through a spark gap formed between the center and ground electrodes.
In operation, a high voltage is applied by an ignition coil to the spark plug, thereby breaking down the electrical insulation of the spark gap to generate sparks between the center and ground electrodes.
Moreover, Japanese Patent Application Publication No. JPH0680313 discloses a spark plug which has a voltage-constant element embedded in the insulator. The voltage-constant element has a characteristic that its electrical resistance decreases when a voltage higher than or equal to a threshold voltage is applied to it. The voltage-constant element is implemented by an avalanche diode or a varistor. The voltage-constant element is located within the metal shell and electrically connected between the secondary side of the ignition coil and the metal shell in parallel with a spark-discharge path that includes the center electrode, the spark gap, and the ground electrode. Consequently, with the voltage-constant element, it is possible to prevent a voltage higher than or equal to the threshold voltage from being applied across the spark gap. As a result, it is possible to reduce variation in the discharge voltage of the spark plug.
However, the spark plug disclosed in the above patent document may involve the following problems.
In operation, a distal part of the spark plug, which is inserted in a combustion chamber of the engine, will be heated to a high temperature by the heat generated by combustion of the air-fuel mixture in the combustion chamber. Moreover, the metal shell is generally made of a metal material having a relatively high heat conductivity. Therefore, when the distal part of the spark plug is heated, the heat of the distal part will be transmitted to the entire metal shell, thereby heating the entire metal shell also to a high temperature. Accordingly, the voltage-constant element, which is located within the metal shell, may be subjected to a high temperature which exceeds the heatproof temperature of the voltage-constant element. Consequently, the threshold voltage of the voltage-constant element may be changed and, in the worst case, the voltage-constant element may be damaged and become unable to normally function. As a result, it may become impossible for the spark plug to generate stable sparks in the spark gap.